Voltage controlled oscillators (VCO's) have long been used as components within phase locked loops (PLL's) and synthesizers, which in turn have had wide applicability, particularly in the area of clock frequency generation. PLL's for example, are commonly used in a feedback configuration to generate an output clock signal having a desired output frequency that is derived from an input reference signal. More specifically, PLL's/synthesizers are often used in microprocessors and in wireless transmitters and receivers to achieve desired operating/output frequencies based upon relatively low frequency input signals.
With the continued push towards miniaturization of electronic circuits and devices coupled with the demand for increased operating speeds of those devices, operating noise has become a significant and limiting factor in the design of such circuits and devices. In particular, flicker noise has become a dominant source of noise in silicon based metal oxide semiconductor field effect transistor (MOSFET) devices at low frequencies. In fact, flicker noise often limits the achievable dynamic range for baseband/low-frequency circuits and is a critical design constraint in radio frequency (RF) circuits. In VCOs, device-level flicker noise gets up-converted to phase noise. This can result in adjacent channel interference in communication systems.
Flicker noise in MOSFETs is due primarily to the random trapping and detrapping of charge carriers near the Si—SiO2 interface. This carrier trapping/detrapping process acts to modulate the channel carrier density and causes fluctuations in the surface mobility of the charge carriers. Because of the slow time constants of the oxide traps and the surface states, the noise spectral density of flicker noise is inversely proportional to frequency, and therefore is commonly called 1/f noise.
Many VCOs typically use p-channel metal oxide semiconductors (PMOS transistors) cross-coupled to n-channel metal oxide semiconductors (NMOS transistors), as well as an NMOS tail current source and an on-chip inductor and varactor. Although there can be a variety of factors contributing to the generation of phase noise in VCOs, flicker noise in the tail current source and cross-coupled MOS devices has been found to be the dominating contributor to noise. Additionally, transceiver systems built using typical VCOs either receive interference from adjacent channels, or have a high residual bit error rate due to integrated phase noise outside the PLL/synthesizer loop-bandwidth.
Although a variety of noise suppression techniques have been attempted in the past, these approaches all require additional components or processing steps thus increasing cost and design time.